A conventional liquid crystal display device has a TFT substrate and a counter substrate disposed opposite to the TFT substrate. In the TFT substrate, pixel electrodes, thin film transistors (TFTs) and the like are arranged in a matrix form. In the counter substrate, color filters and the like are arranged at locations corresponding to the pixel electrodes of the TFT substrate. A liquid crystal is interposed between the TFT substrate and the counter substrate. Then, an image is formed by controlling the transmittance of light of the liquid crystal molecules for each pixel.
In order to increase the production efficiency of the liquid crystal display device, plural TFT substrates are formed in a mother TFT substrate, and plural counter substrates are formed in a mother counter substrate. The mother TFT substrate and the mother counter substrate are bonded together by a sealing material to form a mother panel including plural liquid crystal cells. The individual liquid crystal cells are separated from the mother panel by scribing or other suitable methods.
The TFT substrate has a large number of pixels each having a TFT and a pixel electrode. Further, a large number of scan lines, image signal lines and the like are formed in the TFT substrate. Because of this configuration, defects such as disconnection and short circuit are likely to occur in TFTs, pixel electrodes, and various lines. If such defects are found in the state of the mother TFT substrate, the following process will no longer be necessary for the defective cell. Further, if a large number of defective TFT substrates are present in the mother TFT substrate, the mother TFT substrate itself will no longer be conveyed to the post process.
JP-A No. 341377/2002 describes a technology for testing a TFT substrate to detect defects in the state of the TFT substrate. In other words, with the technology described in JP-A No. 341377/2002, it is possible to test the presence of a defect in all pixels by using two testing short bars for gate lines, using three testing short bars for image signal lines, and inputting different testing signals to each of the short bars.
The technology described in JP-A No. 341377/2002 uses two test lines for scan lines, and three test lines for image signal lines. In other words, it is necessary to form an area for the test lines in the mother TFT substrate in order to perform the test in the state of the TFT substrate. Depending on the content of the test, the number of test lines is increased and the area of the test lines is increased. As a result, the number of TFT substrates to be formed in one mother TFT substrate is reduced. This leads to an increase in the production cost of the liquid crystal display panel.
The liquid crystal display device is divided into two types based on the materials from which TFT is formed, namely, poly-Si and a-Si. TFT formed from poly-Si is the type called LTPS (Low Temperature poly-Si). In the LTPS type, a driving circuit can be formed on the outside of the display area in the TFT substrate. The driving circuit can be used to detect pixel defects, as well as line defects such as disconnection and short circuit. More specifically, in the TFT of LTPS type, pixel defects and line defects are detected by measuring the size of the storage capacity formed in the pixel. Thus, in the TFT of LTPS type, it is possible to test defects in the state of the TFT substrate, without causing a problem of the area of the test lines.
In the case of the TFT of a-Si type used in large liquid crystal display panels, such as TV panels, each pixel is large. Thus, it is possible to test defects in the pixels and lines without providing different test lines and test terminals. In other words, in the large TFT substrate, all the pixels are turned ON to detect defects in the pixels and lines, for example, by scanning the display area by electron beams and by testing the potential of the surface of each pixel. It is also possible to detect the pixel defects by providing a liquid crystal pack seal on the surface of the display area of the TFT substrate, instead of scanning the display area by electron beams.
However, in the case of the TFT of a-Si type used in small and medium sized liquid crystal display panels, the size of each pixel is small. Thus, it is difficult to apply the test method used in the large liquid crystal display device as described above. In this case, it is necessary to provide lines and terminals for testing in the state of the TFT substrate. If the test is not performed in the state of the TFT substrate, such lines and terminals for the test may not be necessary. If the area of the lines and terminals for the test is increased, there is a problem that the number of TFT substrates to be obtained from one mother TFT substrate is reduced.